In magnetic recording media, as demands for higher-density recording have lately been raised, ferromagnetic metal powdery materials comprised principally of such metals as iron, nickel and cobalt has come into common use in place of those ferromagnetic powdery materials which had conventionally been in use. Particularly in video tape, with the utilization of a method of shortening the recording wavelength or narrowing the track width, extremely high-density video recording has become essential, so that video tape comprising a ferromagnetic metal powdery material in place of the conventional iron oxide-type ferromagnetic powdery material has come into use. In the case where a ferromagnetic metal powdery material is used, it is known that by smoothing the magnetic layer surface, even higher-density video recording can be performed and also the electromagnetic conversion property of a magnetic recording medium can be improved.
However, if the magnetic layer surface is smoothed, particularly while video tape is running, the contact friction resistance between the magnetic layer and the apparatus system increases, and as a result, the magnetic layer of a magnetic recording medium tends to be damaged or peeled off. Especially in video tape, there are cases where its magnetic layer is put under such a severe condition as in the still mode, and an increase in the coefficient of friction of the magnetic layer causes the life of the magnetic layer specially in the still mode to be shortened. Accordingly, improvement of the endurance of the magnetic layer of video tape while running is unrgently needed.
As a conventional measure to improve the running endurance of a magnetic layer, there is proposed a method of adding an abrasive (hard particles) such as corundum, silicon carbide, chromium oxide or the like to the magnetic layer.
The above-mentioned friction resistance is determined according to the product of the coefficient of friction multiplied by the contact area of a contact surface. Therefore, the addition of an abrasive to a magnetic layer to roughen its contact surface to thereby reduce the contact area of the contact surface which is to come into contact with a guide pole or the like enables to lessen the friction resistance. For example, Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) Nos. 130234/1982, 161135/1983, 53825/1982 and 2415/1983 disclose examples of the use of inorganic powdery abrasives, most of which examples specify the use of abrasives limited to those of certain particle sizes. There also are other proposals for the use of carbon black in lieu of the aforementioned inorganic powdery materials, one of which proposals is found in, for example, Japanese Patent Examined publication No. 17401/1977. This is aimed at both the antistatic effect based on the conductivity of carbon black and the surface roughening effect due to its particles, but is disadvantageous in respect that such effects are not always adequately obtained and it rather deteriorates not only the runnability but also the electromagnetic conversion property.
For example, there are known those techniques to improve separately individual characteristics such as the conductivity, friction (runnability), surface characteristic, and wear resistance (as described in Japanese Patent Examined Publication No. 15052/1981 for the conductivity, Japanese Patent Examined Publication No. 20203/1978 for the wear resistance (runnability), and Japanese Patent O.P.I. Publication No. 51025/1981 for the surface characteristic (electromagnetic conversion property). However, in these techniques of the prior art, the dispersibility of magnetic powder is poor and the running endurance of the medium is inadequate, and the oil absorption of carbon black itself is as high as to tend to form a structural construction to increase the viscosity of a magnetic coating liquid, thus deteriorating the physical property of the coating liquid, whereby the electromagnetic conversion property is deteriorated.
There are also known other techniques to use in combination two or more kinds of carbon black different in the nature in order to satisfy the above-mentioned respective characteristics at the same time (as described in Japanese Patent O.P.I. Publication Nos. 5426/1984 and 16141/1984, and Japanese Patent Examined Publication Nos. 20203/1978 and 9041/1979). However, the techniques disclosed in these publications are still not sufficient to satisfy the above characteristics at the same time.
Further, in the case where an abrasive is to be added to a magnetic layer for the purpose of improving the running endurance of the magnetic layer, a fairly large amount of the abrasive has to be added in order to produce satisfactory results of the addition. The magnetic layer to which has been added a fairly large amount of the abrasive, however, causes a magnetic head and others to conspicuously wear down, and runs counter to the purpose of improving the electromagnetic conversion property by smoothing the magnetic layer, so that this method is not considered acceptable.
And, in video tape, the high-density recording, which is made with its recording wavelength shortened or with its track width narrowed, on the other hand, increases the occurrence of drop-out. The increase in the occurrence of the drop-out can be reduced to some extent by making the magnetic layer surface super-smooth or mirror-like smooth. However, providing the magnetic layer with a mirror-like surface is accompanied by the problem that the running endurance must be improved.